The present invention relates in general to he sensing of edges in various industrial processes and, in particular, to a new and useful method and apparatus for measuring the distance to an edge, seam, or interface of a workpiece, and which uses one or two EMATs.
It is known to use an electromagnetic acoustic transducer (EMAT) to inspect a weld. U.S. Pat. No. 5,439,157 to Geier et al. describes an automated butt weld inspection system which employs an EMAT to generate shear horizontal (SH) waves for detecting defects in butt welds. In accomplishing that task, the commercial embodiment of that system, known as a TEMATE.RTM. inspection system, employs an inductive proximity sensor to detect the presence of the edge of the steel plate. More particularly, the proximity sensor is used by sensing when the steel plate is under the proximity sensor, scanning towards the edge of the plate, and sensing the point at which the steel is no longer present to indicate the edge of the plate. The location of the edge using such an inductive proximity sensor typically requires several seconds, and its accuracy is somewhat limited, albeit sufficient for the system disclosed in the '157 patent.
Reflected ultrasound has also been used to automatically control the production of a weld seam. U.S. Pat. No. 4,588,873 to Fenn et al. describes the detection of weld seams, material edges, and the molten weld pool interface using conventional ultrasonic test methods for the purpose of controlling the welding process. Specifically, it describes the use of conventional ultrasonic surface waves for weld seam detection and tracking.
One advantage which EMATs possess over all other ultrasonic sensing techniques is the fact that EMATs do not require couplants or gels between the EMAT sensor and a surface of the workpiece under inspection.
Detection of the edge of a material during welding or other processing is often a necessary and integral part of the process. For instance, during automatic welding, tracking of the seam between the two components being welded is necessary for proper execution of the process. Other processes, such as steel forming and cutting, require accurate location of the work piece edges during the process.
Currently, edge or seam tracking is usually performed with the aid of a laser or by other optical methods. A light beam is transmitted to the part. If the beam strikes the component, there is a return beam. If the beam passes by the edge or seam, there is no return beam. Multiple beams may be used to provide full coverage of the area in which the seam or edge is located; alternatively, a beam may be swept across the area to locate the edge or seam, indicated by the change in the returned beam. These optical methods must provide precise measurements of the edge location, typically +/-0.010" or better, in a process environment that may include welding, cutting, or grinding. Maintaining a clean and clear optical path in this environment is often difficult. In addition, the surface finish of the material dictates the quality of the reflected beam. In some cases, a rough surface may scatter the beam and not allow a strong enough signal to be returned. In other cases, a very smooth surface may provide a beam reflection that is difficult to process due to beam strength or beam scattering.
A need remains for a convenient and effective technique for determining the position of an edge or seam for use in various welding, metal cutting, and other processes.